The issue happened during 2022-April-12,14,22 [i.e. 5 months ago], before doing the upgrade to 15.4 [June,2022] so from that date I am not using 15.3 repos. Such instances still happen [ with 15.4 repos], but it seems that after some time [days] these incidents magically disappear. Sometimes, using a mirror is worse that delaying the maintenance a couple of hours/days. As far as I am concerned, this ticket can be closed [unknown/undetected cause]. ND